config: document --[no-]show-names

config: document --[no-]show-names

These options were introduced in 4e513890008 (builtin/config:
introduce "get" subcommand, 2024-05-06) but not documented here.
Use the description from the source code.

Document this option and the negated form according to the current
convention.[1]

`--show-names` is also the default when `--get-regexp` is given.  But
don’t mention it here since all the deprecated modes are quarantined in
the “Deprecated Modes” section.

[1]: https://lore.kernel.org/git/xmqqcyct1mtq.fsf@gitster.g/

Acked-by: Patrick Steinhardt <redacted>
Signed-off-by: Kristoffer Haugsbakk <redacted>
Signed-off-by: Junio C Hamano <redacted>